She will have to be honest and transparent. This is not the time to make mistakes. Consult with an experienced immigration attorney.
Guerra Saenz, PL--Immigration Attorneys (954) 434-5800. This answer is of a general nature and should not be relied upon as final, nor is it intended as legal advice.
Please take your sister to an immigration lawyer and discuss all of the issues presented. There is much much more to this matter than filing for deferred action.